On 1/27/25 11:21 AM, Andres Freund wrote:

quoted

That's precisely what io-uring fixed buffers do :)

I looked at using them at some point - unfortunately it seems that there is
just {READ,WRITE}_FIXED not {READV,WRITEV}_FIXED. It's *exceedingly* common
for us to do reads/writes where source/target buffers aren't wholly
contiguous. Thus - unless I am misunderstanding something, entirely plausible
- using fixed buffers would unfortunately increase the number of IOs
noticeably.

Should have sent an email about that...

I guess we could add some heuristic to use _FIXED if it doesn't require
splitting an IO into too many sub-ios. But that seems pretty gnarly.

Adding Pavel, he's been working on support registered buffers with
readv/writev.

I dimly recall that I also ran into some around using fixed buffers as a
non-root user. It might just be the accounting of registered buffers as
mlocked memory and the difficulty of configuring that across
distributions. But I unfortunately don't remember any details anymore.

Should just be accounting.

-- 
Jens Axboe